• When I used to wash my hair with a two-for-one shampoo and conditioner, I would wash it twice, which is completely unnecessary, because that's the point - it's one bottle. Because I used to have the shampoo, that's one, conditioner, one. I did the same thing, even though it was two-in-one. I was corrected in my mid-20s by my girlfriend at the time, who laughed at me and said how stupid that was.